1. Field of the Invention
The present invention relates to a defective pixel specifying method and a defective pixel specifying system for a semiconductor device having an image sensor function. The present invention also relates to an image correcting method and an image correcting system for an image read by a semiconductor device having an image sensor function.
2. Description of the Related Art
Various kinds of sensors are developed and put into practice accompanying technology advancement of late. These sensors are used mainly to convert text and image information on paper into data for personal computers. Most of those sensors are semiconductor devices having an image sensor function.
Examples of the above semiconductor devices include digital still cameras, scanners, and copying machines. Digital still cameras are used as replacements for conventional silver film cameras, and have area sensors in which pixels are arranged two-dimensionally. Scanners and copying machines are used as means for reading text and image information on paper, and have line sensors in which pixels are arranged one-dimensionally.
Scanners can be roughly divided by their reading methods into three types; (1) sheet feeding type, (2) flat bed type, and (3) pen type (handy type). The sheet feeding type fixes an image sensor unit of a scanner and the original is moved along by a sheet feeder to read the original. The flat bed type fixes the original on glass and an image sensor unit is moved under the glass to read the original. The pen type (handy type) reads the original when a user moves an image sensor unit on the original.
Scanners of the above three types all employ optical systems. Flat bed type scanners read images finely and therefore often employ demagnification optical systems. A lens used in a demagnification optical system has a long focal distance and, therefore, the distance between a subject and an image sensor unit is large, resulting in a large-sized semiconductor device.
In order to make sheet feeding type and pen type (handy type) scanners portable, the devices have to be small in size. Accordingly, nonmagnification optical systems are employed in many cases. A nonmagnification optical system has a rod lens array interposed between an image sensor unit and a subject. The rod lens array is a bunch of plural rod lenses having a distributed index of refraction. The rod lens array forms an image at 1:1 and therefore has a short focal distance to make the distance between the image sensor and a subject small.
Manufacturers of scanners recommend purchasers of their products to conduct calibration before starting reading a subject.
Calibration is recommended for the following two reasons.
Firstly, a subject is not irradiated uniformly with light from a light source in a scanner. As described above, a lens such as a demagnification optical system or a rod lens array is used in a scanner. Light from the light source provided in the scanner irradiates a subject through those lenses. Accordingly, the intensity of light that irradiates a subject may vary between different areas of the subject.
Secondly, fluctuation in characteristic between pixels of the image sensor can be corrected by calibration. The fluctuation corresponds to a slight difference in signal value read by pixels when the scanner reads a subject that has identical information all over its surface. The fluctuation between pixels causes a difference in signal value outputted from a photoelectric conversion element even when light from the light source irradiates the subject at the same intensity. In most cases, the fluctuation in characteristic between pixels does not change with time.
Thus, calibration on a purchased scanner before starting reading a subject is recommended. In fact, some of scanners on the market contain in their packages calibration sheets having the same sizes as their effective reading range. Calibration sheets are white plastic sheets. Preferably, calibration sheets are untransmissive, solid and plastic sheets. It is also preferable for calibration sheets to have flat surfaces with no hole or dent.
After a calibration sheet is read, all pixels should read identical information. However, information actually read may vary from the two reasons given in the above. Therefore, information when the white sheet is read is stored in a program in the scanner or other devices or media. Then, each time a subject is read, correction is made based on the stored information. Once calibration is conducted, the information is stored in a memory or the like and it is not necessary to repeat calibration.
The method of calibration differs from one semiconductor device to another. For instance, a scanner uses a calibration sheet for calibration. A digital still camera is sold with calibration software included in the package. Then, calibration is conducted using the software. With a digital still camera, a picture is taken through a lens and sometimes the image is slightly distorted. Distortion is measured through calibration. A correction value for distortion of the lens is calculated and inputted to a program of the digital still camera to reduce the influence of the distortion as much as possible.
A semiconductor device having an image sensor function is provided with a pixel portion that has a plurality of pixels. Each of the pixels has a photoelectric conversion element and one or more transistors for controlling the photoelectric conversion element.
Semiconductor devices having an image sensor function are roughly divided into CCD type and CMOS type. CMOS type semiconductor devices are further classified into passive semiconductor devices to which amplifying transistors are not mounted and active semiconductor devices to which amplifying transistors are mounted. An amplifying transistor has a function of amplifying an image signal of a subject read by a photoelectric conversion element.
An active semiconductor device has, in addition to an amplifying transistor as the one described above, semiconductor elements such as a sensor selecting transistor. Accordingly, the number of elements in one pixel is large. When elements in one pixel are increased in number, the yield in manufacturing the semiconductor device lowers.
As a result, it is very difficult to obtain a semiconductor device having no defective pixel. When forming a semiconductor device, a semiconductor device sometimes fails to form a photoelectric conversion element in a pixel, or one of plural transistors for controlling the photoelectric conversion element, properly. A pixel having a failed element cannot operate normally and therefore is incapable of reading the image of a subject correctly. When a semiconductor device having a defective pixel displays an image of a subject read, the defective pixel is often shown as a white dot or a black dot on the screen. Thus the defective pixel on the screen is very noticeable and keeps the semiconductor device from displaying the accurate image of the subject read.